The reason why AutoRX works so well in these older engines is that they are full of deposits from running Group I, solvent refined oils. If you run ARX in an engine thats used a high end sylube there is very little stuff for the ARX to clean up.
By the way, ARX works just fine with PAO/Ester oils. I ran some with Amsoil in an old motor just to see and it cleaned up nicely. Of course I don't know if this is partially a result of the esters in the Amsoil formulation or strictly from the ARX....
TS